1. Field of the Invention
The present invention relates to a curable resin composition containing a (meth)acrylic monomer, which is useful for bonding adherends made of reinforced fibers, metal and/or cotton, preferably for bonding adherends made of reinforced fibers, more preferably for bonding adherends made of aramid fibers. Particularly, it relates to an adhesive composition which is useful for assembling a speaker employing a damper made of aramid fibers.
2. Discussion of the Related Art
The demand for a cold quick-curing adhesive which is curable at a normal or room temperature in a short period of time, tends to increase year after year with a view to labor-saving, resource-saving and energy-saving. Heretofore, as a cold quick-curing adhesive, a two-pack type quick-curable epoxy adhesive, an anaerobic adhesive, an instantaneous adhesive or a second generation acrylic adhesive (SGA) has been known.
The two-pack type quick-curing epoxy adhesive is designed so that a main agent and a curing agent are weighed, mixed and coated on an adherend, whereupon the adhesive cures by the reaction of the main agent and the curing agent. However, such a two-pack type quick-curing epoxy adhesive has had a drawback that if weighing or mixing of the main agent and the curing agent is inadequate, the bond strength is likely to be remarkably low, and even if weighing and mixing are adequately carried out, the peel strength and impact strength tend to be low.
The anaerobic adhesive is designed so that the adhesive composition is pressed between adherends to cut off air for curing. However, it has had a drawback that if a part of the adhesive composition is squeezed out at the time of pressing, such a squeeze-out portion will be in contact with air and will not be cured. Further, it has another drawback that it is not curable when a clearance between adherends is large.
The instantaneous adhesive usually comprises cyanoacrylate as the main component and is excellent in the operation efficiency. However, it has had a drawback that the peel strength or impact strength is low. Further, the moisture resistance or water resistance is poor, whereby the range of its application is limited.
SGA is a two-pack type. Nevertheless, it requires no accurate weighing of the two packs, and it cures at room temperature in a few or a few tens minutes even when weighing or mixing is inadequate, in some cases, simply by contacting the two packs without requiring accurate weighing of the two packs. Accordingly, it is excellent in the operation efficiency, and yet SCA provides high peel strength or impact strength, and curing of the squeeze-out portion is also good. For this reason, it is widely used.
However, such SGA has had a drawback that the adhesion to fibers or cloth is poor, and it tends to hardly adhere to e.g. cloth or paper to be used as a material for the three parts of a speaker i.e. a cone, a damper and a voice coil.
Usually, a speaker has a construction as shown in FIG. 1. Namely, a frame 4 is bonded to a field system comprising a bottom plate 1, a ring-shaped magnet 2 and a ring-shaped top plate 3 which are laminated and bonded. To the peripheral portion of this frame 4, the peripheral portion (the cone edge) 7 of a cone 6 is bonded. To the center portion of this cone 6, a voice coil 8 is bonded. The intermediate portion of this voice coil 8 is held by a damper 9, and the lower portion of the voice coil 8 is fitted on the center portion of the bottom plate 1. A dust cap 10 is bonded to the center upper surface of the cone 6.
Particularly, bonding of the three parts of the speaker comprising the cone 6, the voice coil 8 and the damper 9 is as shown in FIG. 2. The voice coil 8 has a construction such that a paper tape 11 is wound on the outer circumference of the voice coil 8, and a coil 12 is wound on the outer circumference of the paper tape 11 (in the Figure, the portion identified by a circle is the bonding portion of the three parts of the speaker, and the three parts of the speaker are bonded by an adhesive composition 13).
A speaker is assembled by bonding a hard adherend such as a metal or magnetic material and a flexible adherend such as paper or cloth. In the bonding of the hard adherend and the flexible adherend, particularly when a damper 9 or a cone edge 7 made of paper or cloth is bonded to a frame 4 by an adhesive composition 13, it is required that the damper 9 or the cone edge 7 will not lift from the frame due to warpage from the time before curing after coating and press-bonding.
To satisfy such requirements, JP-B-60-19789 proposes an adhesive composition suitable for a porous or expandable material. By such an adhesive composition, bonding to cloth or paper useful for e.g. the three parts of the speaker, has been made possible.
However, in recent years, there has been an increasing demand for a high power speaker or a speaker to be mounted on an automobile. In the case of a high power speaker, a large quantity of electric current flows in the voice coil, thus leading to heat generation, and in the case of the speaker to be mounted on an automobile, the speaker itself is required to have heat resistance as the interior temperature becomes high during the operation of the engine or in the summer time.
Accordingly, the material to be used for the damper 9 of the speaker is now being changed from paper or cloth to aramid fibers having high heat resistance. Here, the term "aramid fibers" is an abbreviation of "aromatic polyamide fibers".
Therefore, an adhesive composition excellent in bonding to the aramid fibers, has been desired.